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Why doesn’t my schema that has a default property actually set the default on my instance?¶

The basic answer is that the specification does not require that
default actually do anything.

For an inkling as to why it doesn’t actually do anything, consider that none
of the other validators modify the instance either. More importantly, having
default modify the instance can produce quite peculiar things.
It’s perfectly valid (and perhaps even useful) to have a default that is not
valid under the schema it lives in! So an instance modified by the default
would pass validation the first time, but fail the second!

Still, filling in defaults is a thing that is useful. jsonschema
allows you to define your own validator classes and callables, so you can easily create a IValidator that does do
default setting. Here’s some code to get you started. (In this code, we add
the default properties to each object before the properties are validated,
so the default values themselves will need to be valid under the schema.)

fromjsonschemaimportDraft4Validator,validatorsdefextend_with_default(validator_class):validate_properties=validator_class.VALIDATORS["properties"]defset_defaults(validator,properties,instance,schema):forproperty,subschemainproperties.iteritems():if"default"insubschema:instance.setdefault(property,subschema["default"])forerrorinvalidate_properties(validator,properties,instance,schema,):yielderrorreturnvalidators.extend(validator_class,{"properties":set_defaults},)DefaultValidatingDraft4Validator=extend_with_default(Draft4Validator)# Example usage:obj={}schema={'properties':{'foo':{'default':'bar'}}}# Note jsonschem.validate(obj, schema, cls=DefaultValidatingDraft4Validator)# will not work because the metaschema contains `default` directives.DefaultValidatingDraft4Validator(schema).validate(obj)assertobj=={'foo':'bar'}

See the above-linked document for more info on how this works, but basically,
it just extends the properties validator on a
Draft4Validator to then go ahead and update all the defaults.

Note

If you’re interested in a more interesting solution to a larger class of these
types of transformations, keep an eye on Seep, which is an experimental data
transformation and extraction library written on top of jsonschema.

Hint

The above code can provide default values for an entire object and all of its properties,
but only if your schema provides a default value for the object itself, like so:

This means broadly that no backwards-incompatible changes should be made in
minor releases (and certainly not in dot releases).

The full picture requires defining what constitutes a backwards-incompatible
change.

The following are simple examples of things considered public API, and
therefore should not be changed without bumping a major version number:

module names and contents, when not marked private by Python convention
(a single leading underscore)

function and object signature (parameter order and name)

The following are not considered public API and may change without notice:

the exact wording and contents of error messages; typical
reasons to do this seem to involve unit tests. API users are
encouraged to use the extensive introspection provided in
ValidationErrors instead to make
meaningful assertions about what failed.

the order in which validation errors are returned or raised

the compat.py module, which is for internal compatibility use

anything marked private

With the exception of the last two of those, flippant changes are avoided, but
changes can and will be made if there is improvement to be had. Feel free to
open an issue ticket if there is a specific issue or question worth raising.